Engineers create new nano-fiber tougher than Kevlar

Scanning electron microscope (SEM) image of a DWNT-polymer fiber which exhibits high strength and toughness. The inset shows a transmission electron microscope (TEM) image of a well ordered bundle of DWNTs which are the constituents that make up the macroscopic fibers (Image: Northwestern University)

A new high performance fiber that is better at absorbing energy without breaking than Kevlar has been created by the U.S Department of Defence. While still under development, the material could be used in bulletproof vests, parachutes, or in composite materials for vehicles, airplanes and satellites in the future. The fiber has been engineered from carbon nanotubes spun into a yarn and held together using a polymer. The resultant material is tough and strong while still remaining flexible.

"We want to create new-generation fibers that exhibit both superior strength and toughness," said Horacio Espinosa, Professor of Mechanical Engineering at Northwestern University. "A big issue in engineering fibers is that they are either strong or ductile - we want a fiber that is both. The fibers we fabricated show very high ductility and a very high toughness. They can absorb and dissipate large amounts of energy before failure. We also observed that the strength of the material stays very, very high, which has not been shown before. These fibers can be used for a wide variety of defense and aerospace applications."

The research has been conducted at Northwestern University's McCormick School of Engineering and Applied Science and is part of the Department of Defense Multidisciplinary University Research Initiative program. Espinosa and his collaborators received $7.5 million in funding to develop the material.

To create the new fiber, researchers began with carbon nanotubes. These cylindrical-shaped carbon molecules are known to individually have one of the highest strengths of any material in nature. Previously the largest issue facing materials researchers has been that when nanotubes are bundled together they lose strength because of lateral slippage. To solve this problem a polymer was added to bind them together, and then the resulting material was spun into yarns.

The strength and failure rates of the resulting material was tested using in-situ Scanning Electron Microscope (SEM) testing, which uses a powerful microscope to observe the deformation of materials under a scanning electron beam. "We learned on multiple scales how this material functions," said Tobin Filleter, a postdoctoral researcher in Espinosa's group. "We're going to need to understand how molecules function at these nanometer scales to engineer stronger and tougher fibers in the future."

The result is a material that has a higher ability to absorb energy without breaking than Kevlar, though Kevlar still has a higher resistance to failure. The next step is to study how to engineer the interactions between carbon nanotube bundles and the nanotubes within the bundle itself to increase the strength of the material. The group is currently looking at techniques like covalently crosslinking tubes within bundles using high-energy electron radiation to help better engineer those interactions. A covalent bond is a chemical bond created by the sharing of pairs of electrons between atoms, in this case the carbon atoms of the nanotubes.

"Carbon nanotubes, the nanoscale building blocks of the developed yarns, are still 50 times stronger than the material we created," said Mohammad Naragh. "If we can better engineer the interactions between bundles, we can make the material stronger."